In a medium bowl sift together flour, cocoa and salt.

In a large bowl add the butter and beat on medium speed for 5 minutes until fluffy.

Add sugar and beat about 2 minutes, until very light in color and fluffy, occasionally scraping down the sides of the bowl with a spatula. Add vanilla if using.

Add the flour mixture and combine on low speed, until flour is just incorporated and dough sticks together when squeezed with fingers.

Form dough into a flattened disk; wrap in plastic. Chill until firm, at least 1 hour.

Preheat oven to 325F for 15 minutes. Line 2 baking sheets with parchment paper. I greased it lightly with the butter wrapper itself.

Use a spoon to form the dough into balls and place on the prepared baking sheet. I chilled it for 2 hours and the dough became so hard I was not able to scoop the dough. I had to pinch the dough and roll it in between my palms.

Bake until firm, 20 to 25 minutes, rotating halfway through (that is, moving the baking sheet in the top row to the bottom row and the vice versa) My cookies were ready around 23 minutes.

Cool completely on a wire rack. Dust with cocoa powder just before serving. (I didn't do this)